factory + core: replace jamon with codahale metrics

Codahale metrics is a much more modern and actively maintained solution compared to jamon:
http://metrics.codahale.com/

Thanks to metrics module, one can easily add other monitoring, for instance at db level. It also has more features, like histograms.

With this implementation we loose one interesting data though: last access. Still need to figure out if we can implement it on top of codahale metrics.